Specific Aims: This project attempts to answer 4 questions: (1) What are the activities of Ca++, Na+, K+, and C1- in the alveolar subphase? (2) Does ionic activity in the subphase depend on lung volume? (3) Does cardonic anhydrase contribute to the high H+ activity in the subphase? (4) Do acetylcholine, terbutaline, and oubain change ionic activity there? Methods: We have previously measured the H+ activity in the alveolar subphase by puncturing the subpleural alveoli of the lungs of anesthetized rabbits with a pH microelectrode. In this project, we utilize the same protocol to measure Ca++, Na+, K+, and C1- in the subphase with microelectrodes that are selective for these ions. In addition, we will measure the effects of lung volume and drugs on ionic activity. Finally, we will collect samples of alveolar fluid and measure the total concentration of these elements with electron probe analysis to compare total and ionized concentrations. Long Range Objectives: The surface tension properties of surface active materials in the alveolus have received great attention, but the aqueous subphase portion of the alveolar lining is unexplored. This subphase might greatly alter the ability of the surface active materials to lower surface tension through interactions of subphase ions, such as Ca++ and H+, with lipids and apoprotein in the lung surfactant. Our long range goal is to understand the role of the subphase in the alveolar lining.